1. Field of the Invention
This invention is directed to an acrylate or methacrylate (collectively referred to as (meth)acrylate in this invention) composition that is actinic radiation curable and a waveguide device fabricated with the composition. The (meth)acrylate composition demonstrates excellent coating, fast cure, high photo contrast, low optical absorption loss, high thermo-optic coefficient, and high thermal stability. The waveguide device is used in fiber optic communication networks which use single mode optical waveguides to interconnect various fiber optic devices as well as glass optical fibers.
2. Description of the Prior Art
A waveguide is a planar structure comprising of a high refractive index material in the core and low refractive index materials in the cladding that surrounds the core. The core geometry and the refractive index difference between the core and the cladding determine if the waveguide is single mode and its mode size. The waveguide can be in any form or shape depending its end use.
It is known in the art that a photosensitive composition can be patterned by UV light, e-beam, reactive ion etching (RIE) and lasers to produce waveguide structures. One method used to form waveguides involves the application of standard photolithography processes. A chosen pattern in a photosensitive polymer layer deposited on a substrate is defined with a photo mask using the photolithographic process. Another method used to fabricate waveguides involves the use of RIE. Among the many known photosensitive polymers, (meth)acrylate materials have been widely studied as waveguide materials because of their optical clarity and low birefringence. The details of the prior art are described in U.S. Pat. Nos. 4,609,252; 4,877,717; 5,054,872; 5,136,682; 5,396,350; 5,402,514; 5,462,700; 5,481,385; 6,023,545; 6,114,090, which are incorporated herein by reference.
The waveguides used for telecommunications applications come in a variety of forms and shapes and can be used to produce fiber optic components such as thermo-optic switches, splitters, combiners, couplers, filters, attenuators, wavelength cross connects, channel monitors, and add-drop multiplexers. The fiber optic applications require that the materials and their waveguide devices meet certain specifications such as low optical insertion loss, high thermo-optic coefficient, low birefringence, and high reliability. It is often very challenging and takes a unique material and device approach to meet all the requirements.
To achieve low insertion loss it is necessary to simultaneously realize low absorption and scattering losses of the waveguides in the telecommunication wavelength region of 1,300-1,610 nm and low coupling loss between the waveguides and their pigtailed fibers. To realize low absorption loss it is required to use materials that have low absorption at 1,300-1,610 nm. To realize low scattering loss it is required to use materials and device fabrication processes that allow for homogeneity and minimize stress. Waveguides should have matching optical fibers in both cross section and mode size to realize low coupling loss. To realize the matched cross section and mode size, high contrast materials are utilized to control the waveguide size as well as the capability to control refractive indexes of the materials.
Although the prior art teaches how to fabricate waveguides with photo polymers, practicing of the prior art has not led to devices that meet all the requirements for practical use in telecommunication networks. Typical prior art devices use hydrocarbon materials that have very high absorption loss in the wavelength region of 1,300-1,610 nm. Also prior art devices use materials that have high shrinkage upon curing, leading to high residual stress and hence high scattering loss. All these losses lead to devices that have unacceptably high insertion loss.
Prior art references such as U.S. Pat. Nos. 3,77,9627; 4,138,194; 5,062,680; 6,005,137, which are herein incorporated by reference, have taught that replacing hydrocarbon bonds with fluorocarbon bonds or deuterium-carbon bonds will reduce the absorption loss of an organic material at 1,300-1,610 nm wavelengths. Replacing hydrogen with fluorine in a material also decreases the refractive index of the material. Fluorinated photosensitive materials including (meth)acrylates have been used to fabricate waveguides. However these (meth)acrylates are of low molecular weight and are mono functional. There are several drawbacks to such materials. First of all, the relatively low level of fluorination is insufficient to lower the optical loss to the required level. Second, it is difficult to fully cure mono functional monomers with UV light. The residual monomer will cause reliability and environmental problems. Third, the low molecular weight of the monomers leads to very high shrinkage (up to 20%) upon curing. The high shrinkage causes high residual stress, which leads to such problems as high light scattering and poor reliability. Fourth, the high volatility of low molecular weight monomers impairs production of waveguides. The highly volatile monomers not only contaminate the curing chamber but also make it extremely difficult to achieve consistent material properties including refractive index after curing. Fogging of the photo mask used to pattern the waveguides is also a problem. Fifth, the cured polymers are brittle and the devices made from them crack upon baking or bending. Even microcracks will cause high light scattering. Sixth, the monomers do not have sufficient viscosity to spin coat consistently uniform films with the thickness required to match the cross-section of the optical fibers.
The ability to fabricate waveguides having the required dimensions, contrast and mode size depends on a number of interacting variables. These include the chemical and physical characteristics of the materials used including the viscosity, volatility, shrinkage, functionality, photo reactivity, photo contrast, and refractive index. Therefore there is a need to have a composition that addresses all of the above issues.